Such methods are used to produce fibre-reinforced plastic laminates where hand lay-up or filament winding is undesirable. The fibre reinforcement can be of any known type, such as chop stand or woven fabric mats, multi-axis interlaid scrims, warp-thread reinforced unidirectional performs, single or joined roving bundles, and of any known material, such as glass, Kevlar, carbon or hemp. The fibre reinforcement may be supplemented with pre-fabricated components, such as fibreglass inserts, pultruded rods, etc. and with sandwich core materials such as balsa wood, foam or honeycomb. The plastic or matrix material may be a thermosetting plastic, such as polyester, vinylester, epoxy, etc., or it may be a thermoplastic, such as polyethylene, polypropylene, PVC, ABS, etc. The plastic laminate may be introduced after the completion of the lay-up of the fibre reinforcement, or it may be integrated with the fibre-reinforcement during lay-up, e.g. in the so-called prepreg materials where the fibre-reinforcement is pre-wetted with the plastic laminate in a non-cured but comparatively solid form that can be handled manually and can be cured once in place, or as a thermoplastic interwoven with or laid up along the fibre-reinforcement.
One such method is resin infusion, where fibre-reinforcement is laid up dry in one mould part, another flexible mould part is added following completion of lay-up so as to form a closed mould around the fibre-reinforcement, the mould is evacuated to achieve good consolidation of the fibre-reinforcement under atmospheric pressure, a liquid matrix material is infused to completely fill in all voids between the reinforcement fibres and between the fibres and the mould parts, and after completion of the injection the matrix material is cured, typically by the application of heat to form the completed composite laminate.
In another such method the fibre-reinforcement is manufactured as a combination material where the fibres are interwoven with or surrounded by a thermoplastic. This combination material is laid up dry in one mould part, another flexible mould part is added following completion of lay-up so as to form a closed mould around the fibre-reinforcement, the mould is evacuated to achieve good consolidation of the fibre-reinforcement under atmospheric pressure, and the combination material is heated above the melting point of the thermoplastic, whereby the now liquid thermoplastic completely fill in all voids between the reinforcement fibres and between the fibres and the mould parts, and after completion of this liquid phase the material is again cooled to form the completed composite laminate.
In most of such methods it can be a problem that air can get trapped in the laminate. Air may be trapped in high points due to buoyancy, but it may also be trapped in parts of the laminate where gravity would be expected to assist in the replacement of the air with the matrix material. Such trapping of air may be caused by geometrical conditions or by the reaction during the curing process, e.g. chemical separation or volatile components.
The problem of potential air entrapment has mainly been solved until now by limiting manufacturing to components that can be viewed through a transparent vacuum bag during the process and where the location of air vents in the vacuum bag can be adjusted as a result of observations. This method does not work, however, when manufacturing large structures with partially or completely closed spaces that cannot be accessed during manufacturing.
Various methods have been suggested to reduce the problem of trapped air in laminates.
Document DE 198 13 105 A1 describes a method where the venting area of a breather hole can be expanded with a semi-permeable membrane. Whilst this method can extend the active area of a suction vent, the effect cannot be ensured if one does not know the location of air inclusions. Furthermore, if the vent is placed on an external surface it requires substantial finishing works after moulded.
Document EP 1 181 149 describes a method whereby the entire vacuum bag surface comprises a semi-permeable membrane. This method will remove air from the laminate surface irrespective of where entrapment occurs, but it has the great disadvantage that if the mould surface is on the up side and the vacuum bag surface is on the down side, there is still a large risk of air inclusions close to the surface next to the mould. Due to buoyancy air will tend to move upwards, away from the venting membrane. The method described in EP 1 181 149 also has the disadvantage that it requires the entire surface of one side of the laminate to be covered by costly supplementary materials such as flow-assisting vacuum bags or semi-permeable membranes. Furthermore, for elongated, partially closed structures the removal of the supplementary materials after curing of the laminate may be difficult or impossible, and the supplementary materials may need to be left in place, which in turn could lead to problems over time with accumulation of condensation etc. in voids between the supplementary materials and the laminate.
In U.S. Pat. No. 5,665,301 a method is disclosed for manufacturing a fiber reinforced composite article. A vacuum is connected to the fiber reinforcement in a mold via multiple peripheral self sealing micro porous conduits passing between a vacuum chamber and the mold cavity containing the fiber reinforcement for the article. Resin is injected in the mold. The resin flow moves toward the peripheral self sealing micro porous conduits allowing any residual air or volatiles remaining in the dry fiber reinforcement, due to imperfect vacuum, to be drawn out through the peripheral self sealing micro porous conduits into the peripheral vacuum chamber and exhausted through the vacuum pump.
U.S. Pat. No. 5,304,339 discloses a method of manufacturing a large-sized, thin-walled, elongated molding of fiber reinforced, hardenable synthetic resin where at least one layer of reinforcing fiber is laid against a form-retaining, rigid inner mold part. A flexible outer mold part is placed against the fiber layer. A liquid, hardenable synthetic resin is flowed into the mold cavity to substantially fill the mold cavity. A reduced pressure is induced within the mold cavity to cause the flexible mold part to be tightly drawn against the fiber layer and toward the inner mold part, air to be removed from within the resin, mold cavity and fiber layer and to cause the resin to flow into the fiber layer and mold cavity. Multi-channeled cores may be placed in the so-formed mold cavity to improve the uniformity of resin distribution.